This helper returns a new property descriptor that wraps the passed
computed property function. You can use this helper to define properties with
native decorator syntax, mixins, or via defineProperty().

When passed as an argument, the set function should accept two parameters,
key and value. The value returned from set will be the new value of the
property.

Note: This is the preferred way to define computed properties when writing third-party
libraries that depend on or use Ember, since there is no guarantee that the user
will have prototype Extensions enabled.